Fix tests on windows, where PollSelector may not be defined.

src/gevent/tests/test__monkey_selectors.py

@@ -30,10 +30,12 @@ class TestSelectors(greentest.TestCase):
         self.assertIn('_gevent_monkey', dir(_select))
 
     @greentest.skipUnless(
-        hasattr(select, 'poll'),
+        hasattr(selectors, 'PollSelector'),
         "Needs gevent.select.poll"
     )
     def test_poll_is_default(self):
+        # Depending on the order of imports, gevent.select.poll may be defined but
+        # selectors.PollSelector may not be defined.
         # https://github.com/gevent/gevent/issues/1466
         self.assertIs(selectors.DefaultSelector, selectors.PollSelector)